In self-contained breathing apparatus, used for respiratory protection in hazardous atmospheres and which include a supply of air or oxygen, hand operated valves are frequently used to provide the extra margin of safety against failure of the breathing gas supply. Such failures might be "failure closed" of the supply regulator valves or "failure open" that would lead to premature loss of the breathing gas. In order for the hand valves to be effective as a backup system, it is essential that the user be properly trained in their use, that he not panic as a result of the primary malfunction, and that his hands are free to operate the valves.
In the previously mentioned breathing apparatus a warning system is usually provided to warn when the breathing gas has been depleted to a pre-established level. In most of these systems, a depleted pressure supply causes the operation of an electrical signal device or a fluid-actuated alarm device such as a whistle or bell wherein the fluid which operates the alarm is wastefully exhausted to the outside atmosphere.